Matter in Our Surroundings • Topic 2 of 3

Change of State & Latent Heat

Ice, water and steam are all the same substance — H2O — in three different states. Matter can change from one state to another, and the two factors that drive these changes are temperature and pressure. Understanding how this works is the heart of this topic.

Effect of changing temperature

When we heat a solid, the particles gain kinetic energy and vibrate faster. At a certain temperature this energy is enough to overcome the strong forces holding them in place, and the solid turns into a liquid. This process is called melting (or fusion), and the fixed temperature at which it happens at normal pressure is the melting point. Ice melts at 0°C (273.15 K). On further heating, the liquid particles move fast enough to escape as a gas — this is boiling, and the temperature at which a liquid boils throughout its bulk at normal pressure is its boiling point (100°C or 373.15 K for water). The reverse changes are freezing (liquid → solid, at the freezing point) and condensation (gas → liquid).

The Kelvin scale

Scientists use the Kelvin (K) scale of temperature. To convert: K = °C + 273 (more precisely +273.15) and °C = K − 273. The lowest possible temperature, 0 K (−273°C), is called absolute zero.

Effect of pressure

Increasing pressure pushes particles closer; with cooling, this can turn a gas into a liquid or even a solid. Solid carbon dioxide (dry ice) is stored under high pressure. Because of this, raising the pressure usually raises the boiling point (water boils above 100°C in a pressure cooker), while lowering the pressure lowers it (water boils below 100°C on a high Himalayan peak).

Sublimation

Some solids change directly into vapour without becoming liquid first. This is called sublimation, and the reverse (vapour → solid directly) is deposition. Camphor, naphthalene balls, ammonium chloride and dry ice all sublime.

Latent heat — the hidden heat

Here is something surprising: while ice is melting, even though we keep supplying heat, the temperature stays fixed at 0°C until all the ice has melted. The heat we add is not raising the temperature — it is being used silently to break the forces of attraction between particles. This hidden heat is called latent heat (latent means hidden).

  • Latent heat of fusion: the heat needed to change 1 kg of a solid into liquid at its melting point, with no rise in temperature. For ice it is about 3.34 × 105 J/kg.
  • Latent heat of vaporisation: the heat needed to change 1 kg of a liquid into vapour at its boiling point, with no rise in temperature. For water it is about 22.5 × 105 J/kg.

This is why steam at 100°C causes far more severe burns than water at 100°C — steam carries the extra latent heat of vaporisation, which it releases onto the skin. It also explains why the temperature stays constant during a change of state: all the energy goes into changing the arrangement of particles, not into speeding them up.

Heating curve of water: temperature stays constant during melting (0 C) and boiling (100 C) because of latent heatHeat supplied →Temperature (°C)0100melting (latent heat of fusion)boiling (latent heat of vaporisation)icewatersteam
Solved Examples
1
Worked Example
Convert the boiling point of water, 100°C, into the Kelvin scale.
Solution
  1. Use the formula K = °C + 273.
  2. K = 100 + 273.
  3. = 373 K (more precisely 373.15 K).

Answer: 100°C = 373 K.

2
Worked Example
Convert 300 K into the Celsius scale.
Solution
  1. Use the formula °C = K − 273.
  2. °C = 300 − 273.
  3. = 27°C.

Answer: 300 K = 27°C.

3
Worked Example
Why does the temperature of melting ice stay at 0°C even though heat is being supplied continuously?
Solution
  1. The heat supplied during melting is the latent heat of fusion.
  2. This heat is used to break the forces of attraction holding the ice particles in fixed positions.
  3. Since the energy goes into changing state, not into increasing kinetic energy, the temperature does not rise.

Answer: The supplied heat is absorbed as latent heat to break inter-particle forces, so the temperature remains constant at 0°C until all the ice melts.

4
Worked Example
A burn from steam at 100°C is more severe than from boiling water at 100°C. Explain why.
Solution
  1. Both are at the same temperature, 100°C, so temperature is not the cause.
  2. Steam is water vapour that has already absorbed the latent heat of vaporisation (about 22.5 × 105 J/kg).
  3. When steam touches the skin and condenses, it releases this extra latent heat onto the skin, in addition to the heat from cooling.

Answer: Steam carries the extra latent heat of vaporisation, which it releases on condensing on the skin, so it causes a more severe burn.

5
Worked Example
Name the change of state in each case and state whether heat is absorbed or released: (a) water turning to ice (b) camphor disappearing without melting.
Solution
  1. (a) Water (liquid) to ice (solid) is freezing; the particles lose energy, so heat is released.
  2. (b) Camphor (solid) changing directly to vapour without becoming liquid is sublimation.
  3. Sublimation requires the particles to gain energy, so heat is absorbed.

Answer: (a) Freezing — heat released. (b) Sublimation — heat absorbed.

6
Worked Example
Why does water boil at a temperature below 100°C high up in the mountains?
Solution
  1. A liquid boils when its vapour pressure equals the surrounding atmospheric pressure.
  2. At high altitudes the atmospheric pressure is lower than at sea level.
  3. So the liquid reaches the boiling condition at a lower temperature, and water boils below 100°C.

Answer: Lower atmospheric pressure on mountains lowers the boiling point, so water boils below 100°C.

Key Points

  • Change of state is driven by temperature and pressure: melting/fusion (solid→liquid), boiling (liquid→gas), freezing (liquid→solid) and condensation (gas→liquid).
  • Melting point (ice = 0°C) and boiling point (water = 100°C) are fixed temperatures at normal pressure; convert with K = °C + 273.
  • Raising pressure raises the boiling point (pressure cooker); lowering pressure lowers it (high mountains); high pressure plus cooling can liquefy a gas.
  • Sublimation is a solid changing directly to vapour (camphor, naphthalene, dry ice, NH4Cl); the reverse is deposition.
  • Latent heat is the hidden heat absorbed during a change of state without a temperature rise — latent heat of fusion (ice ≈ 3.34 × 105 J/kg) and of vaporisation (water ≈ 22.5 × 105 J/kg); this is why steam burns are worse.
Quick Check — 5 MCQs
Tap an option to check your answer0 / 5
Q1.The change of a solid directly into vapour is called:
Explanation: Sublimation is the direct solid-to-vapour change, as seen in camphor and dry ice.
Q2.The value of 0°C on the Kelvin scale is:
Explanation: K = degC + 273, so 0 degC = 273 K.
Q3.While ice melts at 0°C, the heat supplied is used to:
Explanation: The latent heat of fusion breaks the inter-particle forces, so the temperature stays constant.
Q4.In a pressure cooker, food cooks faster because increased pressure:
Explanation: Higher pressure raises water's boiling point above 100 degC, so food cooks faster.
Q5.Steam at 100°C causes a worse burn than water at 100°C because steam has extra:
Explanation: Steam releases its latent heat of vaporisation on condensing, adding to the burn.
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